Measurement of envelope delay distortion



Oct. 16, 1956 J. B. MAGGIO MEASUREMENT OF' ENVELOPE DELAY,DISTORTI QN Filed June 7, 1952 ATTORNEY United States Patent O MEASUREMENT F ENVELOPE DELAY DISTORTION John B. Maggio, Summit, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 7, 1952, Serial No. 292,345 7 Claims. (Cl. 324-57) This invention relates to testing circuits and more particularly to circuits and methods for measuring the envelope delay distortion of transmission system.

The quantity known as envelope delay may be defined as the rate of change of phase with respect to frequency experienced in the transmission of a given band of frequencies over a circuit under investigation. Envelope delay distortion occurs as a result of variations in this quantity which corresponds to the slope of the phase versus frequency curve of the transmission system.

It has been found that most transmission systems and especially long-haul coaxial cable and radio relay systems present envelope delay distortion problems particularly when used for television, picture or multiplex telephone transmission. in all such systems there is of course an overall phase shift or delay caused by the nite times required for the transmission of signal information over the relatively large distances involved. This delay cannot be avoided and is not objectionable so long as the system introduces the same delay for all frequency components falling within the band of frequencies to be transmitted. Under these conditions the slope of the phase versus frequency characteristic is constant. Most of these systems however do have a measureable variation across the transmission band in the phase delay or shift with respect to frequency, resulting in envelope delay distortion as defined above. As pointed out in an article entitled The Measurement of Delay Distortion in Microwave Repeaters by D. H. Ring, published in the Bell'Sys'tem Technical Journal for April 1948 beginning at page 247, envelope delay may be measured by determining the change in phase of the transmitted wave which occurs as the carrier frequency changes over a particular frequency interval. This quantity which is the slope of the phase versus frequency curve of the transmission system for the chosen interval is, as pointed out above, a constant only in an ideal system. In a system subject to envelope delay distortion the slopes found by measurement over different elemental frequency intervals will differ at different points in the band of frequencies to be transmitted and the way in which these slopes vary is determinative of the envelope delay distortion of the system.

Methods have long been known for the measurement of the envelope delay distortion characteristic of transmission systems on a point-by-point basis as disclosed for example in Measurement of Phase Distortion by H. Nyquist and S. Brand, Bell System Technical Journal, July 1930, pages 526 and 527. Further, various types of equipment have been developed for automatically measun'ng the delay distortion for all frequencies within a given band. All of these methods and equipments however have required the transmission of a reference quantity with respect to which the phase delay can be determined. In certain apparatus for this general purpose a test signal is sent over the system under test and returned over a second system to the initiating station at which a comparison of the relative phase may be accomplished. In other test equipment a reference wave is sent over a parallel system which introduces a known delay and serves as a basis for comparison. In still other apparatus for this purpose the reference signal is sent over the system under investigation along with a test signal and separated therefrom at the receiving station by relatively complicated techniques which involve the use of additional equipment.

Objects of the present invention are to provide equipment for measuring envelope delay distortion on a socalled straight-away basis which shall eliminate the necessity for the transmission of a reference signal and to reduce the cost and complexity of test equipment for this purpose.

In accordance with the invention there is provided apparatus for producing a test signal comprising a carrier which is frequency modulated by a linear saw-tooth wave to sweep the carrier frequency over the band for which delay distortion information is required and further circuits for modulating this carrier either as to amplitude or frequency by a single tone. This test signal is transmitted over the system to be investigated and is detected at the receiver, the type of detector employed depending upon the type of modulation introduced in the test signal. Circuits are provided for recovering both the saw-tooth component, which serves as a sweep signal proportional to frequency for an indicating oscilloscope, and the single tone component. The latter component is applied to two paths one of which includes a delay line. Comparison of the output from the two paths after attenuation to equalize the output amplitudes of the two paths results in a quantity which is a measure of the slope of relative phase delay as a function of frequency for the frequency interval set by the delay line. This quantity is integrated over the frequency range under investigation to obtain the delay distortion characteristic which may be directly presented on the oscilloscope.

The above and other features of the invention will be described in detail in the following specification taken in connection with the drawing in which:

Fig. l is a schematic diagram in block form of apparatus according to the invention for the straight-away measurement of envelope delay distortion; and

Fig. 2 is a block diagram of elements to be substituted between the points x, y, z of Fig. l to modify the equipment disclosed for use with amplitude modulated systems.

One embodiment of the invention as arranged for use in determining the envelope delay distortion characteristic of frequency modulation systems will be described with reference to Fig. l of the drawings. Here the system under investigation which may be a coaxial cable system, radio relay, or similar system is indicated by the dashed block 10. Apparatus for producing the test signal applied according to the invention appears to the left of system 1G in Fig. l and is located at the transmitting terminal of the system. The remaining apparatus appearing at the right of system 10 is located at the receiving terminal of the system and is employed according to the invention to produce directly a visible indication of the delay distortion characteristic of the system.

Basically the apparatus for producing the test signal includes means for modulating by a single frequency tone a carrier the frequency of which is itself swept linearly across the band of frequencies for which the characteristics of transmission system 10 are to be investigated. In other Words, the test signal comprises a carrier that is frequency modulated by a saw-tooth wave and either frequency or amplitude modulated by a single frequency wave. While such a test signal may be produced in a Variety of ways it is convenient as shown in Fig. l to produce the two modulations separately. Thus a conventional oscillator 12 is employed to produce a carrier shown inFig. l .as having an operating the nominal frequency-of which is related to and is prefi erably .several times .the midb and frequency .of the system under test. The operating frequency of oscillator 12 is swept across a frequency band equal in width to the band Vof tthe .fsystem under'test thy'a saw-tooth wane tof .repetition `rate f. lhis saw-.tooth'wave may beaprodueed'roy conventional linear saw-tooth .wave `generator i4 .ancimay control-the frequency ofcsciilator lZthrough a reactance modulator :16 Yof a well-.known type. Such .saw-'tooth modulation .of 'the carrier sweeps vthe frequency :thereof between limits which may be identified as 'Fi and 'and j this frequency modulated .carrier is appiied .to a mixer Also .applied .to Ymixer clllator 120,1the nominal .frequency (EP3) .of which is equal :toithe vvnieanfrequency '.of oscillator l2 less the midband frequency of transmission system 10.-. This oscillator -is modulated.preferably eat a low v.deviation ratio .by afsingle tone 4oortainedrfrom .an `oscillator' 22 frequency .of

ZODkilocyclesper second.

Although either .amplitude or frequency. modulation may .be employed for the application ofthe :singlefre- Y quency'tone to oscillator 26 it is convenient as' assumed in .Fig l .to employ vfrequency modulation .of oscillator 20.if 'thesystem 10 .under .test is a frequency modulation system since the limiting circuits normally found 'in Vsuch systems. would suppress or .eliminate entirely any amplitude .modulation'occurring in the test signal. In this instance the :modulator 24shown .connected .between single tone generator 22 and oscillator 20.may be a 1re.- actance modulator identical to modulator 16.

.The .modulated output ofroscillator 2'0, comprising a carrier 4F3 frequencymodulatedby the isingletoneioutput of. `oscillator .22, combines in mixer V18 withV .the fre.

quencyfswept output ofaoscillator .12 to rprovide a .test

i Y modulation detector 'or 'discriminator 26. at the outputof which are available both'the single frequency tone l.and

1'8 is the .output .of a vsecond osoutputs of the two paths.

shifterjmay be Aprovided to .introduce a fixed phase dexthe delay network thereby to Y i p 4' second path. The delay'to be is .the .time .required to .sweep signal over the chosen elementary introduced by network 38 .the -frequency of the .test frequency'interval and Vdepeds upon the repetition rate of the saw-tooth sweep wave and upon the frequency interval employed. As

the repetition rate is increased, the delay may be reduced for measurement of phase. lshift over a given frequency interval. .lf rit .is .desired to .increase lthe .precision of measurement'by employing a `smaller 'frequencyintervaL the delay must he correspondingly reduced.

Adirect measure :of the .phase .delay over the interval of the-delaynetwork -may be obtained by comparison -of the outputs of the two paths.v Such a eomparisonjmay'be made bya .conventional phase .sensitive detector 44. i

Either of the so-called phasemeter circuits disclosed YYin Electron Tube Circuits by .Seely, McGraw-Hill,Y 19750,.

beginning at page 483 are satisfactory for this'purpose. To facilitatesuch comparison, in .thesecond path to compensate for anyloss in the'delay line .thus equalizing the amplitudes ofthe signals .atthe Further, an adjustable phase lay. rin :the rpath4 including provide .a zero point .corresponding to a particular chosen frequency in the band transmitted by the system under investigation. Y e

The output of the phase sensitive detector isa measure Y of the slope of thefphase .characteristic for the particular frequency {interval determined by .the time vinterval set by .delay network 3S 4Vand the rate of frequency sweep. lnzthe presenceof distortion this quantity will vary Vover different portions of the transmission Vband andmust .be

' integrated over -the band of fthe system under test (this corresponds tothe periodof electrontube-integratingcircuits known in the art.

In such casethe frequencies speciliedVY Y y characteristic for .the'system the saw-,tooth wave employedforsweepingthe :frequency of :oscillator 12. These Voutputs .are 'separated-..by.lters,` .a :low-pass filter 28 being employed to .selectthe saw- Ytooth wave .for applicationthrough an .amplier toethe Yhorizontal.deilection plates of .a cathode Vrayoscilloscope The use'ofethegrecoveredsaw-tooth wave Vasa `sweep insures-za linear frequency .i scale :and also .automatically compensates for anylixed .time delay betweenethe .sending andxreceiving ends ofthe circuit under 4test.

' The other output Aof the Adetector vcomprisinggthevrecovered'singlefrequency'tone is Aselected by a bandfpass inter-34 andapplied to two separate paths. Thetirst. of these paths includes azphasezshifteriand ardelay network 38 whilethe second includes `anattenuator '.40 Vand one pair of vcontacts of a double-.throw solenoidf operated 'switchdl It will .bennderstood that as--the frequency of the test signal carrier is swept .acrosstheband of the system under investigationfdistortion .occasioned@by-.11n-V equal phase delays offdilerent carrier frequency components will result in'variations in :the relative'phase'of therrecovered single `frequency tone. Y vThusif this "tone is appliedto the two-paths'referredto.above the tone at the output of delay network 37S will have undergone a phase shiftor delay during the .interval introduced by the network which willnot be .enperienced by .the tonelinthe Vtion .by an amplifier 48 the saw-tooth Y wave sweep) to'obtain the 'delay distortion characteristic.V Thisris accomplished by. an integrator. dwhich may infsimpleform comprisefa circuit including a series resistor and .shunt capacitor Yor may comprise one of the more.Y complicated output'of integratingcircnit 46-1s a quantity which during Vthe fsweep of Vthe'test signal varies as the envelope delay introduced bythe system under test. After'amplilca-V thisquantity is applied to theV vertical deflecting plates yof oscilloscope 32,1the horizon- Y tal sweep of .which is synchronized with -the sweepoflthe test'signalas pointed out above. VThere is thus fobtained azvisual representation of .the v.envelope delay distortion The indication appearing on the screen "of voscilloscope 3i) may-be calibrated both as to frequency andas .to delay byiknown techniques. For example,y absorption typewave meters may befintroduced` in tandemfwithgthe transmission path ofthe system under investigation to produce .pips 'during the horizontaldeflection-of thejoscilloscope cor.- responding to'known frequencies in the band of transmission ,system 10. Similarly, intermittently introduced inV one of the two paths between lter 34 and phase Vdetector 44 to produce a shift inthe y oscilloscopeltrace corresponding to arknown delay. .The

extent of this shift may then be measured yand the vertical scale :calibrated directly in :terms of delay. Apparatus for introducing-such a known phase shift isshown in Fig. 'lfan'd :comprises aphase shifter 52 which-may be con- 'nected-l in circuit through switch 42, thesolenoid'of which is equalized by a' low Ifrequency oscillator 50. This switch alternately connects tov one vof'tlie inputs of phase sensitive detector44 orto thesame input through phase .shifter -SZ'introducilngk-a fixed and'known -phasedelav Y Fig. `2is a partial schematic Vratusitotbesubstituted'in the equipment ofiEig. l between the points x, y and z in place "of frequency-rnodulation detector *26, low-passftilter 23 and Vband pass .filterY 34. The rno'glield Ycircuit employs amplitude modulationV of thesweptcarrier .and normally would be employed when attenuator 40 Ais provided The a fixed phase delay may be Y theV output of attenuator k40..directly diagram illustrating appa-Y the transmission system under investigation is itself an amplitude modulated system. In the modied test apparatus modulator 24 is an amplitude modulator and the test signal appearing at the output of mixer 18 may be considered as an amplitude modulated carrier, the frequency of which is swept linearly across the transmission band of system or alternatively as a carrier amplitude modulated by the test tone and frequency modulated by the saw-tooth sweep. Recovery of the single frequency tone and the saw-tooth sweep wave at the receiver station requires the modifications indicated in Fig. 2. Thus the output of transmission system 10 is applied to an amplitude modulation detector 54 which serves to recover the single frequency tone and the output of which is applied through a band pass lter S6 to the two paths referred to above. The output of the system under investigation is also applied to a frequency modulation detector 58 which recovers the saw-tooth wave employed to sweep the carrier frequency in the test signal. The output of frequency modulation detector 5S is applied through amplifier 30 to oscilloscope 32. The system thus modified is equivalent in all other respects to that of Fig. 1 described above and the operation thereof to obtain a direct representation of the envelope delay distortion characteristic is the same.

What is claimed is:

l. In apparatus for measuring over a given transmission band the envelope delay distortion of a transmission system, means for producing and applying to such system a test signal comprising a single carrier the frequency of which is swept across the transmission band to be observed and which is sinusoidally modulated at a chosen frequency, means for detecting the signal transmitted through said system, means for applying the detected signal corresponding to the sinusoidal modulating wave to two paths, one of which includes a delay network, means for equalizing the amplitudes of the outputs of the two paths, means for combining the two outputs to obtain a measurement of the slope of the phase delay characteristic over the interval of said delay network, and means for integrating the combined output to obtain the envelope delay distortion characteristic.

2. In apparatus for measuring over a given transmission band the envelope delay distortion of a transmission system, means for producing and applying to such system a test signal comprising a single carrier the frequency of which is swept across the transmission band to be observed and which is sinusoidally modulated at a chosen frequency, means for detecting the signal transmitted through said system, means for applying the detected signal corresponding to the sinusoidal modulating wave to two paths one of which includes a delay network, an attenuato-r in the other of said paths for equalizing the amplitudes of the outputs of the two paths, a phase detector for accepting the output of the two paths and producing a quantity representative of the phase dierence between the signals in the two paths, and means for integrating the output of said phase detector with respect to time to obtain the envelope delay distortion characteristic.

3. In apparatus for measuring over a given transmission band the envelope delay distortion of a transmission system, means comprising a pair of modulated oscillators, means for frequency modulating one of said oscillators by a low frequency saw-tooth wave, means for modulating the other of said oscillators with a single frequency wave, and means for combining the outputs of the two oscillators to produce a test signal comprising a single carrier the frequency of which is swept across the transmission band to be observed and which is modulated by said single frequency wave; means for applying said test signal to the system to be tested, means for detecting the signal transmitted through said system, separate circuits for recovering from said detected signal the sawtooth wave and the single frequency, means for applying the recovered single frequency wave to two paths one of which includes a delay network, means for combining the outputs of the two paths to obtain a measurement of the slope of the phase delay characteristic over the interval of said delay network, means for integrating the combined output to obtain the envelope delay distortion characteristic, and indicating means responsive to said integrated output and to said reproduced saw-tooth wave to produce a representation of the delay distortion characteristic of said transmission system.

4. ln apparatus for measuring over a given transmission band the envelope delay distortion of a transmission system, means comprising a pair of modulated oscillators, means for frequency modulating one of said oscillators by a low frequency saw-tooth wave, means for modulating the other of said oscillators by a sinusoidal wave, and means for combining the outputs of the two oscillators to produce a test signal comprising a single carrier the frequency of which is swept across the transmission band to be observed and which is modulated by said sinusoidal wave; means for applying said test signal to the system to be tested, means for detecting the signal transmitted through said system, means for recovering in separate circuits and from said detected signal the saw-tooth wave and the sinusoidal wave, means for applying the recovered sinusoidal wave to two paths one of which includes a delay network, means for equalizing the amplitudes of the outputs of the two paths, means for combining the two outputs to obtain a measurement of the slope of the phase delay characteristic over the interval of said delay network, means for integrating the combined output to obtain the envelope delay distortion characteristic, a cathode ray oscilloscope having orthogonal deflection axes, means for applying said reproduced saw-tooth wave to sweep the electron beam of said oscilloscope along one deflection axis and means for applying said integrated output to sweep the said electron beam along the other deflection axis.

5. In apparatus for measuring over a given transmission band the envelope delay distortion of a transmission system, means for producing and applying to such system a test signal comprising a single carrier the frequency of which is swept across the transmission band to be observed and which is sinusoidally modulated at a chosen frequency, means for detecting the signal transmitted through said system, means for applying the detected signal corresponding to the sinusoidal modulating wave to two paths, one of which includes a delay network, means for equalizing the amplitudes of the outputs from the two paths, means for combining the two outputs to obtain a measurement of the slope of the phase delay characteristic o-ver the interval of said delay network, means for integrating the combined output to obtain the envelope delay distortion characteristic, a cathode ray oscilloscope responsive to said integrated output to produce a trace representing the envelope delay distortion characteristic and Calibrating means for said oscilloscope including apparatus for intermittently introducing an additional phase delay in the other of said paths.

6. In apparatus for measuring over a given transmission band the envelope delay distortion yof a transmission system, means comprising an oscillator, means for producing a linear saw-tooth wave of relatively low frequency, means for frequency modulating said oscillator by said saw-tooth wave, a second oscillator having a frequency different from the mean frequency of said first oscillator by an amount determined by the center frequency of the passband of said transmission system, means for producing a sinusoidal wave of predetermined frequency, means for modulating said second oscillator by said sinusoidal wave, and means for combining the outputs of said oscillators to pro-duce a test signal comprising a single carrier the frequency of which is swept across the transmission band to be observed and which is modulated by said sinusoidal wave; means for applying said test signal to said system, means for detecting the signal transmitted .through :said system, .means .fer 'applying the `.entrait ef said .detecting rneans tto a pair ,of .lter elements ene ef which accepte .fregueney Aieernineinente corresponding .t0

' said sinusoidal wavev and ,the other of whichY accepts only those mfrequency components corresponding to said saw-Y tooth wave, means Yfor applying'the output of said iirst filter to twopathsone of which includes a delay networlg,

Y vtilter to the'other o f said dellecting circuits.

. `in apparatus,fonmeasuringiover a given transmission Yband Ytheenvelope delay .distortion of a transmission systern, means comprising aiirst oscillator, means for pro ducing a linear .saw-tooth wave, means 'for frequency modulating said oscillator by said saw-tooth wave, a second oscillator Vhaving a mean frequency differing from the mean frequency of said iirst oscillatorby the center frequency of the ,passband of .the transmission system, means for amplitude modulating said secondl oscillator by a sinusoidal wave of predetermined frequency, and means forcombining the .outputs of said two oscillators to produce Va test signal comprising a single carrier the frequency of which is swept across the transmission band to he observed and which isrnodulated Yby said sinusoidal wavermeans for applying said-test signal .to the system te be tested, .an .amplitude output `of said transmission system, means for applying Y the detected signal corresponding 4to the sinusoidal modulating wave to two paths one of which includes a Adelay network, means for .equalizing the amplitudes of the out'- puts of the twoipaths, means Vfor combining the two outputs to obtain a measurement of the slope of the phase `delay characteristic over Vthe interval of said delay network, means for integrating the combined output to obtain the envelope' delay'distortion, a frequency modula- Y tion vdetector also connected tothe output of said transmis# sion system, a cathode ray oscilloscope .having two Vpairs Y of deflecting circuits, means for applying said output `toone of said pair of deecting circuits, and means for applying ,the output of said frequency modulation detector to the other'said'pair,ofideecting circuits;

tiefer-ences lCited inthe file of this patent UNITED STATES .PATENTS 1,831,881 Potter ;Nov. 17,1931 Y 2,588,376 Fox '--Mar.- .11, :1952 2,617,855 :Etheridgmlr Nov. 1l, 1 952 .2,625,614 .'Schelleng Jan. 13, 195-3 .2,632,792 .Selz Mar. 24, 1953 Y OTHER 'REFERENCES An `Envelope Delay Scanner, Byl-Iunt and Albersheim Bell Telephone System Monograph 1967, published in Proefof thepLnR. E.; vol. 40, pages v454-1459, April, 19,52. i f

modulation deteeter at the integrated Y 

